This invention relates to sheets or flat pads of material which are employed to separate adjacent stacked levels or groups of open containers during transport and prior to filling.
Many types of liquid and other products are furnished in sealed containers composed of metals such as aluminum or steel, or other materials such as glass. A particular example is in the beer industry in which thin wall aluminum cans are used. The light weight cans as furnished to the brewery are pre-painted or labeled and have open tops.
In order to package the cans or other containers for delivery to the filling operation, the empty cans are arranged in a stacked bundle on a rectangular pallet, and the bundle is secured around the perimeter for shipment. Each row or group of equal height cans is arranged in an upright configuration with their sides touching, and each group or row of containers is separated from an adjacent row by a planar rectangular sheet of material referred to as a separator sheet or pad. Successive groups of containers are loaded by sliding them onto and across a separator pad, followed by application of an additional pad. The cans are unloaded in a similar fashion.
Especially in the case of containers for food products and beverages, much effort has been devoted to finding a suitable material for a separator sheet or pad for reasons of sanitation, flavor and safety. Prior art separator sheets have been made of chipboard containing cellulose materials and a binder. These products are porous and fragile and have a limited useful life. Also, cellulose-based products may harbor insects, which may cause contamination of the open top can by virtue of its contact with the separator during packing and shipment. Finally, cellulose and other materials may contribute to off-odors or flavors in the final product in a number of ways. For example, a porous board may absorb solvents or other volatiles generated in the painting process, or the board itself may contain volatiles which contaminate the atmosphere inside the can.
Another important criterion for a reusable separator sheet is the ability of the sheet surfaces to remain free of wrinkles and creases while providing a limited degree of sliding friction during packing and handling. Creases or wrinkles in the sheet are unacceptable because the cans tend to topple over during the loading and unloading operation. At the same time, the sliding friction between the can and the sheet must be low enough to allow loading and unloading. On the other hand, if the surface of the sheets is too slippery, the containers tend to slide out between the liners or separators during transport.
In an attempt to find an acceptable substitute, homogeneous sheets of various polymers, such as polyester and polyvinyl chloride, have been proposed. Polyester sheets have been found to be unsuitable because they are not resistant to tear propagation and also build up a static electrical charge and may create a fire hazard. Vinyl polymers may contain volatile components such as plasticizers which are emitted into the can after packaging, thereby affecting or altering the flavor after filling. Also, some of these materials tend to tear easily, and are either not sufficiently stiff or tend to crack at lower temperatures.